Google font functionality

Hi!

I have added google font functionality for the preview and the Zip package download.

Example:

```FONT: Helvetica, Arial, sans-serif```

If you change it to use a Google Font for example:

```FONT: Dosis, Arial, sans-serif```

and then you click the update button, it will use the first font defined in FONT to fetch google and it will use it for the preview and download.

I hope you like it,

Javi.